Metal nanoparticles such as gold nanoparticles have unique properties that are absent in bulk materials, and therefore, metal nanoparticles are utilized in a variety of fields including colored materials, fluorescent materials, polarizing materials, nanocrystalline materials, electroconductive materials, and the like.
Methods for producing the metal nanoparticles are roughly classified into dry methods and wet methods, and examples of the dry methods include a deposition method. However, special apparatuses are required in this method.
On the contrary, according to wet methods, metal nanoparticles can be obtained by simple operations, and as a representative method, a method of obtaining metal nanoparticles by causing a protective agent and a metal ion to co-exist in a liquid phase and adding a reducing agent thereto, is known. According to this method, the metal nanoparticles are supported on the protective agent, and a metal nanoparticle composite is formed. Therefore, aggregation of metal nanoparticles in a liquid is prevented.
Further, in recent years, there is a demand for an enhancement of the stability of a composite in a liquid, and an increase in the concentration of metal nanoparticles in a metal nanoparticle dispersion liquid containing this composite, from the viewpoint of increasing the usefulness of the composite when the composite is utilized in the colored materials and the like described above. Furthermore, an improvement is desired for the dispersibility in the case where a resin, a crosslinkable compound or the like has been incorporated into such a dispersion liquid, or in the case where this dispersion liquid is used to form a dispersed film.
Also, in order to contribute to these demands, technologies for reducing excess protective agent or reducing agent, or ionic impurities present in the dispersion liquid are known. Regarding such technologies, various methods for purifying dispersion liquids have been suggested, such as electrodialysis (Patent Literature 1); a method of replacing the solvent by ultrafiltration using water (Patent Literature 2); a method of adding a dispersibility lowering agent to a water-soluble dispersion liquid, subsequently settling metal nanoparticles by centrifugation, and removing the dispersing medium (Patent Literature 3); a method of washing a non-water-soluble dispersion liquid with a large amount of deionized water (Patent Literature 4); and a method of adding a large amount of a poor solvent to a non-water-soluble dispersion liquid to settle aggregates, and then separating the aggregates by decantation or centrifugation (Patent Literatures 5 and 6).